Area Forecast Discussion
Issued by NWS Grand Junction, CO

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FXUS65 KGJT 190109

Area Forecast Discussion
National Weather Service GRAND JUNCTION CO
709 PM MDT Thu May 18 2017

Issued at 659 PM MDT Thu May 18 2017

Short range model guidance is showing most precipitation shifting
eastward as the evening and night progresses. This makes sense as
the area of low pressure, and thus best lift, also shifts in this
direction. Significant accumulating precipitation looks to have
ended for the Roan/Tavaputs, Uncompahgre Plateau, and Yampa River
Basin so dropped highlights there. A few showers will persist this
evening and overnight but do not expect any appreciable
accumulations.  Favorable flow keeps some snow going for the
eastern Uintahs so kept highlights there for now. Highlights also
remain for the remaining mountains but a few of them may also come
down early. We`ll be keeping an eye on things and adjust as


.SHORT TERM...(Tonight through Friday night)
Issued at 340 PM MDT Thu May 18 2017

The upper low responsible for significant snowfall across many
mountain areas and northern Colorado valleys is currently centered
somewhere over the Paradox Valley in southwest Colorado with the
overall circulation looking pretty elongated due to strong
dynamics on the northern side. These strong dynamics are causing moderate
to heavy snowfall over the northern and central mountains as well
as the northwest Colorado valleys as overrunning is overtaking
any orographic influence. The upper level flow on the northern
side of the low is from the east which doesn`t tend to favor our
side of the divide. However, these dynamics are strong enough to
overcome the orographic influence. Therefore, the highlites for
the northern and central Colorado mountains were upgraded earlier
this morning to Winter Storm Warnings for 10 to 20 inches of storm
total snowfall, and looks on track. A TROWAL (Trough of Warm Air
Aloft) signature is very evident along the Front Range of Colorado
stretching westward across Wyoming. This is nosing into northern
Colorado which is also allowing cold air to remain trapped among
the northern valleys of Craig, Hayden and Steamboat Springs. This
overrunning of the cold dome is causing efficient snowfall
production as well. So Winter Storm Warnings remain in place for
the northern valleys until midnight tonight. It should be noted
that the intensity should lessen this evening as the upper low
shifts further eastward, pushing the better dynamics and nose of
the TROWAL to the east. Hence, the expiration time for the
northern valleys. Areas south of the upper low including the
Uncompahgre Plateau and San Juans will see more of a scattered
convective nature to the precipitation with periods of moderate to
heavy snow over the mountains and lesser additional snowfall
expected. These areas remain in Winter Weather Advisories through
Friday which are on track.

Regarding freeze potential for the valleys, those that will be
susceptible to those impacts to vegetation look to clear out
somewhat with less cloud cover. This includes the southwest
Colorado valleys of the Cortez area, the Paradox Valley and the
Montrose area. Latest guidance indicates low temperatures tonight
will be in the upper 20s to low 30s for these zones so upgraded
the Freeze Watch to a Freeze Warning. Looking at other sites, like
the Grand Valley, cloud cover will remain in place with
downsloping drainage winds keeping the atmosphere well mixed
overnight. Therefore, temps look to remain above freezing once

As the upper low shifts eastward, it is projected to be over the
central divide mountains by Friday morning with moisture rotating
around the northern portion. Precipitation intensity will lessen
throughout the day on Friday with the northern and central
Colorado mountains being favored in northwest flow. The NAM is
pretty spotty with showers showing a largely convective nature to
the precipitation while the GFS and ECMWF hang onto the showers a
bit longer into Friday afternoon. Largely improving conditions
will be seen from the west with drier air moving in due to a
building ridge of high pressure over the west coast. Thinking
periods of moderate to heavy snow still remain possible over these
mountain areas with lulls from time to time. The current end time
of 6 pm Friday for the winter highlites still looks reasonable to
account for this discrepancy as these lows are sometimes slower
to move out. Some valley fog may develop both tonight and Friday
night due to recent precipitation.

.LONG TERM...(Saturday through Thursday)
Issued at 340 PM MDT Thu May 18 2017

Saturday overall looks like a drier day as the upper low moves
across the northern high Plains. Some showers may linger over the
northwest Colorado mountains due to continued orographics but
intensity looks fairly light. This upper low will move into
Ontario and the northern Great Lakes by Sunday with a few
shortwave disturbances rotating around the periphery, clipping our
northern mountains for a chance of some convective showers
developing each afternoon from Sunday through Tuesday. The ridge
of high pressure will build over the Pacific Northwest by Tuesday
night into Wednesday and slide over the CWA through Thursday,
leading to mostly drier conditions. Still kept some isolated
shower activity over the mountains due to uncertainty in the
models that far out in when it moves the ridge overhead.
Regardless, temperatures will rebound and warm back up to normal
levels by Sunday and continue towards above normal by mid-week.


.AVIATION...(For the 00Z TAFS through 00Z Friday evening)
Issued at 548 PM MDT Thu May 18 2017

With the closed low sitting over Colorado today...unsettled
weather continues to plague the TAF sites. Nearly all TAF sites
will see VCSH with an occasional strike or two of lightning
possible this evening. MVFR will be possible in stronger showers
with LIFR conditions expected to persist at TEX until 02z before
conditions improving. Conditions are expected to improve after
06z...but isolated fog development will be possible overnight.


CO...Winter Weather Advisory until 6 PM MDT Friday for COZ018-019.

     Winter Storm Warning until 6 PM MDT Friday for COZ004-009-010-

     Freeze Warning from midnight tonight to 10 AM MDT Friday for

     Winter Storm Warning until midnight MDT tonight for COZ005.

UT...Winter Weather Advisory until 6 PM MDT Friday for UTZ023.



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